The present invention relates to orthopedic load sensors. More particularly the present invention is related to a fluid chamber for sensors that monitor the load borne by a limb to prevent harmful over-loading of the limb.
Orthopedic physical therapy programs commonly include a training period in which limited weight-bearing use of the affected limb is required for rehabilitation. The amount of weight shifted to the affected limb, however, must be carefully monitored. Of particular concern are those patients fitted with internal or external fixation devices, or joint prostheses, and those patients recovering from repair of the interior cruciate ligament in the knee. Also, research has shown that moderate, intermittent loading of a bone fracture enhances bone growth and shortens the patients' recovery time.
U.S. Pat. No. 3,791,375 discloses a fluid-filled load cell used for sensing the weight applied to a patient's limb to limit the weight borne by that limb during the patient's recovery. The load cell is made of circular, substantially rigid upper and lower plates that are maintained in a spaced relation to each other by a elastic spacer. A load cell is placed beneath the sole and beneath the heel of the patient's shoe, inside pads that approximate the outline of the sole and heel of the patient's shoe, respectively. These pads are then held in place by an ordinary rubber overshoe, which is worn over the patient's shoe.
This device, however, will produce inaccurately low readings for those patients who apply weight on the limb that is not centered over the load cells. These load cells are accurate for forces acting through them, but many patients will occasionally place their weight on one side of the affected foot or, alternatively, balance backward toward the heel or forward toward the toes of the affected foot. In such instances, these loads do not pass through the cells, but through their "spacer" elements, and may not detect the entire load acting on the limb. The resulting failure to detect excessive force can result in injury to the patient.
U.S. Pat. No. 3,974,491 discloses a sensor having a fluid-filled chamber that is a continuous, resilient tube having a circular cross section. This tube is coiled under the heel and sole of a patient's foot, inside a sponge rubber foot pad that is sandwiched between adhesive sheets of a flexible, dimensionally table material such as rubber-coated fabric.
This foot pad does not measure the total load placed on the limb, because (1) a portion of that load is borne by the sponge rubber pad rather than by the tubular sensing chamber, and (2) because the tube is not directly beneath all parts of the foot which the patient can place weight upon (e.g. the side of the foot, or the toes). Thus, this sensing chamber does not provide a reliable measure of the weight borne by the patient's limb.
German Offenlegungsschrift 3 631 923 also discloses a foot pad for measuring forces in a shoe. In this arrangement, the chamber is in the form of either an undulating tube, or a chamber having an internal spacer separating upper and lower sheets. The spacer is in the form of an intermediate layer in the chamber that has holes extending therethrough to define channels and to hold the upper and lower walls of the chamber apart. Neither of these arrangement uniformly support the foot on the pressure sensing fluid, and, when spacers are used, the spacers take up a part of the load.
These references do not consider various safety aspects of the fluid that is used in the chamber, nor do they consider the problem of reactions between the fluid and the material of the chamber.